The air pollutants exist in two major physical phases: particle phase pollutants and gaseous phase pollutants. The particle phase pollutants are pollutants with significant physical sizes including dust particles, airborne bacteria and mold. They may be composed and be bound together by different matters with different compositions. The gaseous phase pollutants are pollutants with simple chemical structures. The size of the gaseous phase pollutants are small from angstrom to nano-scale. Odor molecules and volatile organic compounds are examples of gaseous phase pollutants.
To eliminate the particle phase pollutants or pollutants with significant physical sizes, electrostatic precipitator or high efficiency particulate air filter are always employed. The removal efficiency may reach to 80% to 99.9%. However, it is not always easy to remove the gaseous phase pollutants.
Oxidants such as ozone and hydroxyl radicals are generated from an ozone generator or ionizer. They are very good reactants and can decompose the harmful and small molecular matters. Use of oxidants has been widely used for air purification. However, the rate of air purification depends on the concentration of the oxidants. When the ozone and hydroxyl radical are directly released into the air, their concentrations are immediately diluted. On another hand, these oxidants bombard other non-target molecules in the fluid and become decayed. Therefore, they cannot effectively oxidize the pollutants and organic molecules. Consequently, most ionizers cannot effectively purify the air. Only high concentration of ozone can effectively clean the air. To address this problem, some air purifiers use a catalyst for air cleaning. For example, when a photo-catalyst is being irradiated under a UV lamp, oxidants are generated. An oxidation or reduction reaction of the pollutants occurs.
There are also some air purification methods which involve the adsorption of pollutants into the pores of molecular sieves, based on the size, orientation, shape, and hydrophilicity of the molecule of the pollutants. Within the pores of the molecular sieves, chemical reactions are performed and the pollutants are decomposed. In these air purification systems, the two pollutant phases are drawn into the systems at the same time. Particle phase pollutants are first trapped in the particle filter. The gaseous phase pollutants are drawn into the catalyst filtering core where a chemical reaction occurs for decomposing the gaseous phase pollutants. However, even a high efficiency particulate air filter with a very high efficiency cannot remove all particles phase pollutants. In heavy polluted areas, such as a smoking area, the polluted air drawn into the system containing particle filter and catalyst filtering core results in un-filtered particles being deposited on the surface of the catalyst. This blocks the active sites of the catalyst and hinders the catalytic reaction. Consequently, the gaseous phase pollutants cannot be effectively eliminated. The accumulation of the particle phase pollutants on the surface of the catalyst leads to other problems such as becoming an incubation area for the growth of bacteria. Using such air purifiers which contain the contaminated catalyst filtering core may release the originally collected pollutants (for example, bacteria, odor, dust). The filter needs to be frequently replaced. This is environmentally un-friendly as the unwanted contaminated catalyst filtering core creates a secondary pollution problem.